You might take a look at Banjo-Builder. Even if you find a decent kit, the hints there should be helpful.

Dennis has sort of a mishmash of "how to" links, including how to make a couple of fairly simple (crude?) banjos.

Some previous threads have mentioned banjo kits, I believe. I'll see if I can pull something up, if someone who actually remembers a good source doesn't get here first.

John